The present invention relates to a video signal transient edge enhancing device and method for an image processing system, and more particularly to a video signal transient edge enhancing device and method, which can obtain a clear image in an enhanced edge by properly enhancing the transient edges of video signals having different frequency bands such as TV, VCR, S-VHS, etc., according to the respective frequency bands.
A video signal transient edge enhancing device enables a clear image picture to be watched by enhancing an input video signal, i.e., edge components of luminance signal or chrominance signal which is separated from a composite video signal. Referring to FIG. 1, which shows a conventional video signal transient edge enhancing device, the conventional enhancing device comprises delay elements 1 and 2 for delaying an input video signal for a predetermined time, amplifiers 3, 4 and 5 for respectively amplifying the video signal and the video signals delayed by the respective delay elements, an adder 6 for adding the amplified video signals, an attenuator 7 for adjusting an enhanced level of the added video signal, and an adder 8 for outputting a finally enhanced video signal by adding the attenuated video signal and the video signal delayed by the delay element 1.
The inputted video signal is supplied to adder 6 via -1/2 amplifier 3 and also is inputted to delay element 1 to be delayed for a predetermined time. The output of delay element 1 is inputted to adder 8 and the other delay element 2, and at the same time, is amplified by amplifier 4 to be supplied to adder 6. Also, the video signal delayed for a predetermined time by delay element 2 is amplified by -1/2 amplifier 5, and then is inputted to adder 6.
Here, if the delay time of delay elements 1 and 2 is a 1-line period, the device operates as a circuit for enhancing the input video signal in a vertical direction, and if it is a 1-sample period, the device operates as a circuit for enhancing the input video signal in a horizontal direction. The signal delay, amplification and addition operations, as described above, are a set of differential processes, and as a result of the differential processes, overshoot and undershoot are performed at transient edges (i.e., rising and falling edges) of the input video signal. Accordingly, the edge components within an image are enhanced, and the enhanced degree (level) of the enhanced signal is then adjusted by attenuator 7. The enhanced signal is added to the video signal passed through delay element 1 by adder 8, thereby outputting a video signal having enhanced transient edges.
For instance, since the luminance of an NTSC TV signal has a bandwidth of 4.2 MHz and its edge components correspond to about 2 to 3.5 MHz band, a clear image is reproduced by producing a video signal having enhanced transient edges, using a differential circuit (or peaking circuit) for enhancing the frequency band component.
However, a current TV processes as an input signal a VCR signal or an S-VHS signal as well as a TV signal transmitted from a broadcasting station. The bandwidths and transient levels of the respective video signals such as VCR, TV(NTSC), S-VHS, etc., are different. As shown in FIGS. 3 and 4, the video signals have different frequency bands according to their signal processing types (or circuits) and thus have different frequency bands of transient edge in the respective signals. Accordingly, it is required that the transient edge be accurately enhanced corresponding to the respective frequency bands.
However, in the conventional video signal transient edge enhancing device for use in processing a TV (NTSC) signal having a bandwidth of about 4 MHz as shown in FIG. 3, the frequency region to be enhanced is 2 to 4 MHz. Accordingly, if this enhancing device is applied to a VCR signal having a bandwidth of about 3 MHz or an S-VHS signal having a bandwidth of about 5 MHz, frequency band excluding the edge region is enhanced, thereby reproducing an unnatural and low quality image. That is, when it is applied in the VCR signal, high frequency signal is enhanced, thereby outputting an image having much noise, and when it is applied to the S-VHS signal, frequency signal being lower than the edge region is also enhanced, thereby outputting an unnatural image. When an ideal signal is inputted as shown in FIG. 4, the widths of the respective transient edges of TV, VCR, and S-VHS signals increase because of limited bandwidth. This means that there is a difference in the frequencies of edge regions of the respective video signals. Also, since human vision has a feeling of enhanced transient edge as if overshoot or undershoot is performed in the sharp transient edge, as shown in FIG. 2, it can be known that additional enhancement of the transient edge is not needed.